Myoclonus and a number of other rapid involuntary movements have been difficult to classify clinically. Clinical and physiological analysis of a continuing series of patients has led to new classifications and pathophysiological insights. A new application of our neurophysiological work has been the differential diagnosis of myoclonus versus tic disorder. Positron emission tomography (PET) studies of patients with palatal myoclonus have revealed that the inferior olives are hyperactive. PET studies of patients with hemiballismus have revealed that hypoactivity of the ipsilateral striatum may be a responsible functional lesion. PET studies of patients with essential tremor have revealed that the inferior olives are hypermetabolic during the tremor, but not when the patients are at rest. Task specific focal dystonias of the hands such as writer's cramp and pianist's cramp have been analyzed and a number of physiological characteristics have been defined. There appears to be diminished ability to control the fingers independently and gating of somatosensory evoked potentials with voluntary movement is abnormal. The spasms themselves have been characterized into different patterns. Abnormalities of the blink reflex have been identified in dystonic disorders. We have verified this in a number of our own patients and are now applying this test to the patients with focal hand cramps. These patients have also been studied with a specific test for analysis of reciprocal inhibition in the arm. Reciprocal inhibition is diminished in the arm which shows the focal dystonia, but not in the arm which is not dystonic. This finding of a physiological abnormality in focal hand cramps is the most objective laboratory abnormality seen in these patients, and is now the strongest evidence against the psychogenic origin on the disorder.